Refactoring 018 - Replace Singleton

Breaking Free from the Evil Singleton

Maximiliano Contieri

Nov 05, 2024

TL;DR: Refactor singletons to reduce coupling

Problems Addressed 😔

High coupling
Difficult testability
Multi-threading issues

Related Code Smells 💨

Code Smell 32 - Singletons

Code Smell 22 - Helpers

Code Smell 25 - Pattern Abusers

Steps 👣

Identify the singleton
Locate all references to its getInstance() method
Refactor the singleton to a standard class
Inject it as a dependency

Sample Code 📖

Before 🚨

public class DatabaseConnection {
    private static DatabaseConnection instance;

    private DatabaseConnection() {}

    public static DatabaseConnection getInstance() {
        if (instance == null) {
            instance = new DatabaseConnection();
        }
        return instance;
    }

    public void connect() { 
    }
}

public class Service {
    public void performTask() {
        DatabaseConnection connection = 
            DatabaseConnection.getInstance();
        connection.connect(); 
    }
}

After 👉

public class DatabaseConnection {  
    // 1. Identify the singleton 
    public void connect() { 
    }
}

public class Service {
    // 2. Locate all references to its getInstance() method.
    private DatabaseConnection connection;

    // 3. Refactor the singleton to a standard class. 
    public Service(DatabaseConnection connection) {
        // 4. Inject it as a dependency.
        this.connection = connection;
    }

    public void performTask() {
        connection.connect(); 
    }
}

DatabaseConnection connection = new DatabaseConnection();
// You can also mock the connection in your tests

Service service = new Service(connection);
service.performTask();